Ordering of Poly(3-hexylthiophene) in Solutions ... - ACS Publications

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Ordering of Poly(3-hexylthiophene) in Solutions and Films: Effects of Fiber Length and Grain Boundaries on Anisotropy and Mobility Nabil Kleinhenz,† Nils Persson,‡ Zongzhe Xue,‡ Ping Hsun Chu,‡ Gang Wang,‡,∥ Zhibo Yuan,† Michael A. McBride,‡ Dalsu Choi,‡ Martha A. Grover,*,‡ and Elsa Reichmanis*,†,‡,§ †

School of Chemistry and Biochemistry, ‡School of Chemical & Biomolecular Engineering, and §School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ∥ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, Shanghai 201620, P.R. China S Supporting Information *

ABSTRACT: Long-range ordering emerges in poly(3-hexylthiophene) (P3HT) solutions during time-dependent aggregation. Here, aggregation of P3HT in chloroform solution was induced by ultrasonication, aging, and combinations thereof. UV−vis spectroscopy and polarized optical microscopy demonstrated that long-range ordering in the solution and subsequently the solid state depends on assembled P3HT fiber length, as determined by film atomic force microscopy. Ultrasonication induced the formation of fibers that were relatively short compared to those obtained through aging. As a result, ultrasonication afforded isotropic solutions and films, whereas aging afforded anisotropic solutions and films. The impact of fiber length and anisotropy on macroscopic charge transport performance was evaluated using an organic field-effect transistor (OFET) architecture. Both aged and sonicated solutions exhibited charge carrier mobilities that were an order of magnitude higher than that obtained for pristine samples. Aging of sonicated solutions enabled semiconducting thin films with significantly higher mobilities (1.5 × 10−1 cm2 V−1 s−1) than those of either solution processing technique. Furthermore, the results indicate that grain boundary morphology has a significant impact on macroscopic charge carrier mobility. Grazing incidence wide-angle X-ray scattering demonstrated that the combined sonication/aging method affords a solidified film where the semiconductor exhibits a highly edge-on orientation. The results suggest that the nucleation and growth of aggregates can be controlled via solution processing methods and thus may allow the manipulation of active layer orientation, crystal packing density, and crystallite size. The investigation provides insight into the conjugated polymer solution process parameters that impact polymer ordering and aggregation in solution and resultant thin films for high-performance organic electronic devices.

1. INTRODUCTION There is much interest in the development of semiconducting conjugated polymers for lightweight, flexible, large area, potentially cost-effective and solution-processable organic electronics applications such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic photovoltaics (OPVs).1−5 To improve organic semiconductor performance, increased molecular ordering of the polymer chains, primarily through π−π interactions in the film, is desired to facilitate efficient charge transport.6−8 Studies using poly(3hexylthiophene) (P3HT) as a model system demonstrated that ordered thin-film morphologies are advantageous for transport.9,10 A variety of methods to enhance both intramolecular and intermolecular ordering in conjugated polymers, which depend upon both predeposition (in solution) and postdeposition techniques, have been developed to achieve the apparently desired morphologies for increased charge carrier mobilities.11−17 © XXXX American Chemical Society

Perfectly crystalline solids are expected to afford the most favorable charge transport performance; however, macroscale crystalline solids are rarely achieved in polymeric semiconductors. The materials are generally semicrystalline, where the presence of grain boundaries act as charge traps that limit mobility over a long-range.8 Hence, liquid crystalline materials, with an intermediate degree of ordering in between that of an isotropic fluid and a perfect crystal, were investigated as a means to achieve long-range order while limiting the effects of grain boundaries.8,18 Some studies have investigated the formation of a conjugated polymer lyotropic liquid crystalline (LLC) state for this purpose.19−21 For instance, P3HT in trichlorobenzene (TCB) was shown to form a LLC phase at the edge of an evaporating droplet of solution.21 Received: March 22, 2016 Revised: May 6, 2016

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DOI: 10.1021/acs.chemmater.6b01163 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Experimental diagram of P3HT solution processing by (a) aging, (b) sonication then aging, and (c) aging then sonication. (d) Spin-coating to form thin films. (e) Bottom-contact OFET geometry employed for testing electrical properties.

a change in color from orange to dark brown/purple indicated the self-assembly of P3HT.22 As shown in Figure 1b, a portion of the as prepared solution was first sonicated for 2 min, causing an almost immediate color change to dark brown/ purple. The sample was then characterized and allowed to stand in a capped vial for the desired interval. Figure 1c depicts a solution that was first aged for 96 h and then sonicated for 2 min. All characterization data for both solutions and films refer to the indicated duration of aging and/or sonication of the solution before active layer thin-film deposition by spin-coating. Figure 2a presents changes in the UV−vis absorption spectra as a function of solution processing conditions. As the solution aged, an increase in the 0−0 and 0−1 vibrational peaks at 2.0 and 2.2 eV (620 and 564 nm), respectively, was observed, indicating increased P3HT chain interactions through π−π stacking induced aggregation.26 Figure 2b shows the evolution of aggregation.27 Aging led to a sharp initial increase in the percent aggregation followed by apparent saturation at 22% after 96 h. Although sonication for 2 min induced some aggregate formation (8%), the highest level of aggregation was achieved for solutions that were both sonicated and aged (all greater than 24%). The highest level of solution aggregation (32%) was achieved by sonication followed by 96 h of aging. Considering aggregate formation under a nucleation and growth model, the enhanced level of aggregation observed for the combined process most likely arises from accelerated growth due to the larger number of nucleation sites, or “seeds”, provided by initial sonication.25 Disentanglement of polymer chains induced by sonication could also enable faster diffusion of free polymer species to aggregate growth fronts.28 To probe P3HT solution-state orientational ordering, the solutions were loaded into capillaries and viewed through crossed polarizers, as shown in the POM images (Figure 2c). With time and increased aggregation levels, the solutions became birefringent and exhibited increased long-range ordering (see Figure S1 for modulation in transmitted intensity as capillaries are rotated and for linear dichroism images). Birefringence and long-range ordering, however, do not depend merely on the percent of aggregates. Note that the sonicated then aged 96 h solution as well as the 96 h aged then sonicated solution both appear isotropic (dark) with no apparent long-

More recently it was demonstrated that P3HT in TCB undergoes time-dependent self-assembly (aggregation) to afford birefringent fluids with long-range order in capillary tubes.22 The percent of aggregated polymer, as calculated from UV−vis spectroscopic measurements, rose with time, as did the orientational order parameter as determined by polarized micro-Raman spectroscopy. The results suggested a need for a minimum degree of aggregation for the emergence of liquid crystallinity. Additionally, increased aging time led to more ordering. The study points to potentially significant implications for solution processing of conjugated polymer thin films with long-range ordering. Alternatively, the semiconducting polymer can also be induced to aggregate by a variety of techniques, including ultrasonication.23−25 In this study, the effects of aggregation on P3HT thin film charge transport characteristics were evaluated for sonicated and aged P3HT/chloroform solutions. UV−vis spectroscopy, polarized optical microscopy (POM), polarized Raman spectroscopy, and atomic force microscopy (AFM) analysis indicated that long-range ordering in solution and resultant films depends on average self-assembled P3HT fiber length. Compared to long fibrillar structures obtained through aging, ultrasonication induced formation of relatively short fibers. Solutions and films prepared via the two approaches were anisotropic or isotropic, respectively. Significantly, aging of the sonicated solution afforded thin films having superior charge transport characteristics (0.15 cm2 V−1 s−1). Grazing incidence wide-angle X-ray scattering (GIWAXS), mobility, UV−vis and anisotropy results suggest that in addition to fiber length, grain boundary morphology is an essential factor that determines macroscale charge transport performance.

2. RESULTS AND DISCUSSION 2.1. UV−Vis and Polarized Optical Microscopy. Figure 1 presents a schematic representation of the P3HT solution processing techniques used here. P3HT/chloroform solutions (5 mg/mL) were prepared at 70 °C, after which the orange solutions were allowed to cool to room temperature. Upon reaching room temperature, solutions were injected into vials. For pure aging (Figure 1a), the capped vial was simply left to stand at room temperature in the dark for the desired duration; B


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Figure 2. (a) Sample UV−vis spectra of P3HT solutions processed by aging and sonication techniques. (b) Percent aggregates calculated from the solution UV−vis spectra. (c) Polarized optical microscopy images of solutions processed as indicated before filling 1 mm ID capillaries (45° to polarizer). (d) Sample UV−vis spectra of films spin-coated from aged and/or sonicated solutions. (e) Free exciton bandwidth calculated from the film spectra using the Spano model.

increased average conjugation length. W is calculated from the intensities of the (0−0) and (0−1) transitions (calculated from fittings of the experimental UV−vis spectra in Figure S3) in eq 1:

range order within the capillary despite some of the highest observed levels of aggregation. Thus, other factors must contribute significantly to the formation of birefringent fluids; one key parameter may be the average length of the P3HT nanofibers that form in solution, as discussed below. Figure 2d shows that both aging and sonication increase the intensity of the low energy (0−0) absorption bands in the respective thin-films obtained from the solution-processed conjugated polymer. Thus, the nanofibers formed in solution survive the spin-coating process.23 Further, according to Spano’s model, these vibronic bands are related to the free exciton bandwidth (W), which correlates with intrachain ordering along an individual polymer chain.29 A decrease in W indicates both increased intramolecular ordering and

⎫2 ⎪ 1 − 0.24W / E p ⎪ I0 − 0 ⎧ ⎬ =⎨ ⎪ I0 − 1 ⎪ ⎩ 1 + 0.073W /Ep ⎭

(1)

where I0−0 and I0−1 are the intensities of the (0−0) and (0−1) transitions, respectively, and Ep is the vibrational energy of the symmetric vinyl stretch (taken as 0.18 eV).29 Figure 2e shows the calculated free exciton bandwidths for the films. Compared to the fresh (pristine) P3HT case (0 h) with a W of 123 meV, C


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Figure 3. (a) AFM phase images of spin-coated films (insets are optical images of films as viewed between crossed polarizers), (b) average fiber lengths calculated from the AFM images using FiberApp software30 with standard error from 70 fibers, and (c) Raman anisotropy values calculated from polarized Raman spectra on films as the ratio of the highest intensity to the intensity obtained when the sample was rotated by a further 90°. Raman measurements were taken from three locations on the sample to obtain the average anisotropy (error bars are standard deviation from the mean).

birefringent films, suggesting that solution-state ordering can be transferred to the solid state even through a spin-coating process. As shown in Figure 3b, the average fiber lengths within the respective films were quantified by AFM image analysis using the FiberApp software.30 Fibers were chosen for analysis so that their apparent start and end points fell within the boundaries of the image; 70 fibers were sampled per image. All three films that appear isotropic by POM (0 h, sonication, and 96 h followed by sonication) had the three shortest average fiber lengths. Thus, fiber length appears to be closely related to solution and solid-state anisotropy. This could be explained on the grounds that longer rodlike objects more easily align themselves in a dispersion31 and that alignment is more likely to be preserved in the solid state. However, an additional degree of ordering appears to be present in solid-state AFM images: Longer fibers appear in bundles of parallel orientation. This phenomenon was previously observed by Wang et al. in films cast from solutions of P3HT subjected to microfluidic preprocessing.17 Oriented fiber aggregates could be attributed to physical bridges between neighboring fibers formed by the

decreases in W were observed with longer aging times (down to 55 meV for 96 h of aging), as a result of sonication (62 meV), and especially as a result of combinations of aging and sonication (all below 40 meV). These significant reductions in exciton bandwidth point to increased conjugation along the polymer backbone within films cast from solutions processed by sonication and aging. 2.2. Atomic Force Microscopy, Polarized Optical Microscopy, and Polarized Raman Spectroscopy of Solidified Thin Films. Figure 3a displays atomic force microscopy (AFM) phase images of films obtained from the respective processed solutions deposited onto OFET devices. The insets are images of films on glass as viewed through crossed polarizers (inset images are of points approximately two-thirds of the way between the center and the edge of the sample, with the sample held at the angle producing the brightest image; see Figure S2 for multiple angles). Although the pristine solution afforded an isotropic (dark) film with no visible fibrillar morphology, aged solutions resulted in films containing long nanofibers; some films appeared birefringent. Comparing Figures 2c and 3a, all birefringent solutions afforded D


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Figure 4. (a) 2D GIWAXS diffraction patterns of five representative samples, (b) calculations of Herman’s orientation factor and grain size, and (c) calculations of lamellar stacking distance and π−π stacking distance for all samples.

incorporation of long P3HT chains between fibers or to thermodynamic effects favoring the formation of ordered fibrillar domains at the liquid−air interface during processing.32 All solutions that were isotropic formed isotropic films, with the exception of the sample that was sonicated before 96 h of aging; this sample was isotropic in its solution state but birefringent as a thin film. It is conceivable that the sonicated then 96 h aged sample became birefringent in the solidified film because the film drying process may facilitate additional ordering of the long fibers beyond that which could be achieved in the fluid state. Note that nanofiber growth is expected to continue as solvent evaporates and the film solidifies as per the nucleation and growth model.25,33 The differences between “sonication followed by aging” and “aging followed by sonication” are also evident in the AFM images of Figure 3a. The short fibers present in the 96 h aged then sonicated sample indicate that long fibers present before sonication (as seen in the 96 h case) broke into smaller fragments upon sonication resulting in many shorter fibers with isotropic orientation. As shown in Figure 3c, in order to obtain a more quantitative comparison of the solid-state alignment, Raman anisotropy measurements, an effective probe of chain orientation in conjugated polymer thin films, were performed.34 Raman spectra were measured with parallel polarizers as the films on silicon substrates were rotated on a stage in order to observe the change in P3HT CC stretching peak Raman intensity calculated using Lorentzian spectral fits. The Raman anisotropy values were calculated as the average ratio of the highest intensity to the intensity obtained when the sample was rotated by a further 90°. Comparison of Figure 3b,c reveals that longer average fiber lengths tend to result in Raman anisotropy values significantly greater than 1, with the exception of the sonicated then 96 h aged sample.

Comparing the Raman anisotropy and POM results, samples that appear isotropic by POM (dark) also appear isotropic by Raman (anisotropy value near 1), again with the exception of the sonicated then 96 h aged sample that showed birefringence and alignment by POM but which had only a 1.1 Raman anisotropy value. This is likely due to the fact that the microRaman spectroscopy method employed involves a laser spot size of approximately 1.7 μm (based on a numerical aperture of 0.55 and a laser wavelength of 785 nm), which is probing a region 3 orders of magnitude smaller than that in the millimeter-scale POM images. Therefore, for this sample, the surprisingly low Raman anisotropy value may indicate that many shorter fibers generated by sonication may have been relatively randomly oriented on the smaller scale of microRaman, despite the longer fibers (several micrometers) generating longer range ordering observable by POM. Overall, the anisotropy results confirm that fiber length, solution-state ordering, and resultant solid-state ordering are closely related. It is unclear whether fiber length drives orientational order or vice versa or whether both of these structural features are simply the result of the underlying crystallization process and its thermodynamic driving forces.35 2.3. Film Crystallinity. To probe thin-film structural properties further, films fabricated from solutions processed with the aging and sonication techniques were investigated by grazing incidence wide-angle X-ray scattering (GIWAXS) (Figure 4). Diffraction patterns of five representative samples are shown in Figure 4a (see all GIWAXS results in Figure S4a). Diffraction peaks associated with molecular layering (100, 200, 300) along the qz direction and corresponding to the interlamellar spacing of P3HT (∼16.1 Å) normal to the substrate were apparent in all films.36 Longer aging times resulted in higher intensity peaks, indicating enhanced crystallinity. Although the pristine P3HT film exhibited E


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Figure 5. (a) Average field-effect mobilities of P3HT films spin-coated on bottom-contact OFET devices, calculated at a VD = −80 V. Four devices were characterized for each set of conditions. Error bars show standard deviations from the mean. (b) Transfer characteristics of the P3HT films.

stacked aggregates. The sonicated solution had a hole mobility that was similar to that of the 96 h aged solution (average of 0.112 ± 0.003 cm2 V−1 s−1). The sonicated and then aged 96 h sample afforded the highest mobility (0.149 ± 0.006 cm2 V−1 s−1), representing a 46% increase over that of the 96 h aged sample, a 34% increase over that of the sonicated material, and an 11-fold improvement over that of pristine P3HT devices. In contrast, a solution aged for 96 h and then sonicated afforded a low mobility value of 0.022 ± 0.001 cm2 V−1 s−1, likely due to an increase in grain boundaries that are not well-connected, created when the P3HT fibers formed during aging are broken during the sonication process (note the short fiber lengths observed in Figure 3). It would be mechanistically informative to identify the structural features that best explain the trends in mobility. It is tempting to cite Herman’s orientation factor (Figure 4b) and the percent aggregates in solution (Figure 2b) as the principal structural features driving electrical performance because both display plateau-like behavior for aged devices and reach a maximum (32% and 0.78, respectively) for the highest performing device. An edge-on grain orientation relative to the dielectric substrate, indicated by the high value of Herman’s orientation factor, has long been known to facilitate charge transport by placing the lattice vectors with the highest degree of electronic delocalization in the plane of active charge transport.37 From UV−vis, a high fraction of aggregates in solution should correspond with the formation of more π−π stacking interactions, which are also beneficial to charge transport.26,40 Plots of mobility as a function of eight measured structural parameters are provided in Figure S6. However, the sonicated device and the 96 h aged then sonicated device seem to contradict this analysis. The former has the second highest mobility yet the second lowest Herman’s orientation factor and second lowest percent aggregates in solution. In fact, every quantifiable structural feature of the sonicated device other than (100) grain size and film exciton bandwidth suggest it should be one of the two worst devices of the set. In contrast, the 96 h aged then sonicated device displays the second lowest mobility, although it still demonstrates the third highest percent aggregates in solution and average characteristics from GIWAXS. Perhaps this can be rationalized by considering the impact of grain boundaries. Sonication clearly breaks preformed P3HT

relatively isotropic patterns in the (010) arcs along the in-plane (qxy) axis, films from aged solutions (especially 48 h and onward) and films from combined sonication and aging gave stronger (010) arcs with crystallites that are well organized preferentially in an edge-on orientation, likely due to improved P3HT π−π stacking.37 The full width at half-maximum (fwhm) of the (100) diffraction peaks was used to calculate the crystalline domain sizes via Scherrer’s equation in reciprocal space (see figure S4b for sample fitting and equation).38 The grain size (domain size) tended to increase with aging time in the case of pure aging, but decreased with aging time when the solution was sonicated before aging. The d010 spacing, or the π−π stacking distances, were calculated using Bragg’s law and the positions of (010) diffraction peaks. The π−π stacking distance did not change significantly from sample to sample (approximately 3.65 Å). The d100 spacing, or the lamellar stacking distance, varied slightly, ranging from 16.57 Å (24 h aged) to 16.02 Å. Because semiconducting polymer electrical properties are closely tied to the crystal orientation, the Herman’s orientation factor, f H, was calculated for all samples from the first-order alkyl stacking (100) peaks (Figure 4b). Values of f H range can range from −0.5 (indicating a lattice plane oriented perfectly parallel to the substrate, or face-on) to 1 (denoting crystal planes oriented perfectly perpendicular to the substrate, or edge-on); an f H of 0 indicates randomly oriented structures.39 The sample sonicated followed by 96 h of aging gave rise to the highest f H of 0.79, suggesting a highly oriented microstructure of primarily edge-on character, which is expected to facilitate efficient charge transport in the plane of the substate.37 Comparison of Figures 2b and 4b reveal a similarity in the trends of Herman’s orientation factor and percent aggregates in solution, which can be rationalized as follows: A higher number of π-stacked fibers lying flat on the substrate would indeed yield a higher proportion of edge-on thiophene units. 2.4. Charge Carrier Mobility. Figure 5a presents charge carrier mobilities obtained from films spin-coated onto bottomcontact OFET devices, calculated from the I−V transfer curves shown in Figure 5b. All samples prepared from simply aged solutions exhibited approximately 6−8-fold improved mobility compared with that of the pristine sample (e.g., 0.013 ± 0.006 cm2 V−1 s−1 versus 0.102 ± 0.015 cm2 V−1 s−1 for 0 and 96 h of aging, respectively), which was due to the formation of πF


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particles interferes with fiber alignment in the fluid state. Also, films fabricated from sonicated solutions did not show a high degree of anisotropy. Importantly, semiconducting thin-film anisotropy was not the most important determinant of mobility. Analysis of all the mobility results points to the significance of grain boundaries in addition to fiber length. Films composed of fibers induced by sonication alone may still have a sufficient number of free chains to effectively connect grains, whereas short fibers generated from sonicating an aged solution are plagued with grain boundaries that are likely poorly connected and plagued with vacancies. The results indicate the need for careful consideration of grain boundary morphology in polymeric semiconducting films. Given that solution phase P3HT fiber length and solid-state ordering play significant roles in electronic performance, further alignment methods such as blade coating and substrate rubbing could be employed with appropriate solution-phase semiconducting polymer aggregation techniques to take full advantage of lyotropic liquid crystalline characteristics. This study provides significant insight into the role of long-range ordering in conjugated polymer fluids and thin films along with subtleties that impact ordering and conjugation length within solidified films. The mechanistic analysis will create new research opportunities that will enable future high-performance organic electronic devices.

nanofibers into smaller segments, as evidenced by the difference in fiber length between the 96 h aged sample and the 96 h aged then sonicated sample (Figure 3b). Despite extensive fiber breakup, the percent aggregates in solution remains high (Figure 2b); it stands to reason that five 200 nm P3HT nanofibers would absorb nearly as much incident radiation as a single 1000 nm nanofiber. The breakup of fibers in solution, however, would create many additional grain boundaries along the interface through which charge must flow. Salleo et al. suggest that the “softness” of grain boundaries is a significant factor in determining the mobility of a given material sample.41 It is conceivable that the small fibers in the 96 h aged then sonicated device are poorly connected and plagued by grain boundaries with deep electronic traps or even vacancies, whereas the grain boundaries between the small fibers in the just sonicated device are filled favorably by the freestanding polymers in that solution. The high percentage of aggregates in solution for the 96 h aged then sonicated device suggests that fewer freestanding polymer chains are available to favorably fill in the amorphous grain boundaries in the resulting film. Additionally, aging likely favors the folding of long, dangling polymer chains into the crystalline lattice of the nanofibers, preventing these polymers from becoming effective “tie chains” after sonication breaks the fibers into smaller pieces.42 The above discussion is indicative of the shift toward the characterization of grain boundary morphology in the study of polymeric transistors. The electrical performance of crystalline silicon, after all, is defect-driven.43 Although semicrystalline polymeric transistors will never approach crystalline inorganics on the spectrum of defect density, it is nonetheless worthwhile to consider the structure−property relationship from this standpoint. Recent modeling efforts have begun to focus more on the effect of intergrain connections, and it is acknowledged that inherent paracrystalline disorder places a limit on overall device performance as well.40,44 It is likely that the structure of grain boundaries and polymer crystal defects will play a more important role in research as both characterization tools and our understanding of these materials improve.

4. EXPERIMENTAL SECTION 4.1. Materials. Regioregular P3HT was purchased from Rieke Metals, Inc. (catalog no. RMI-001EE, regioregularity = 96% HT, Mw = 71 kDa, PDI = 2.2) and used without further purification. Chloroform (anhydrous) was purchased from Sigma-Aldrich and used without further purification. For POM studies, borosilicate glass capillaries of dimensions 0.1 × 1 × 50 mm3 were purchased from Vitrocom, Inc., and used without any further surface treatment. Silicon wafers (ndoped) were purchased from Rogue Valley Microdevices, Inc., with a thermally grown 300 nm thick SiO2 dielectric for OFET fabrication. 4.2. Solution Processing of P3HT. A chloroform−P3HT solution (5 mg/mL) was prepared in a 20 mL borosilicate glass vial by heating to 70 °C for approximately 30 min until fully dissolved. The solution was then left to cool to room temperature. At room temperature (but still orange), a syringe was used to take up the solution and inject it back through an 18 guage needle before aging or sonication took place. Vials were wrapped in parafilm and stored in a dark cabinet for aging. For sonicated solutions, the sealed vial was ultrasonicated for 2 min using a tabletop bath-type ultrasonicator (Bransonic 2510, 40 kHz, 130 W) filled with tap water. All solution processing was done in air. 4.3. OFET Fabrication and Characterization. OFET devices with bottom-gate, bottom-contact geometry were fabricated to test the electrical properties of the P3HT films. Highly n-doped silicon wafers with a 300 nm SiO2 dielectric layer were used as the substrate. The doped silicon was used as the gate electrode with the SiO2 dielectric on top. Source and drain contacts were fabricated using a standard photolithography-based lift-off process in a clean room, followed by Ebeam evaporation of the Cr (3 nm) adhesion layer and the Au (50 nm) contacts. Prior to spin-coating the processed P3HT solutions, the OFET substrates were cleaned by sonication in acetone for 10 min followed by rinsing with acetone, methanol, and isopropanol, followed by cleaning in a UV−ozone chamber (Novascan PSD-UV) for 15 min to remove organic contaminants. P3HT solutions were spin-coated on the cleaned devices at 800 rpm for 30 s in air (WS-650MZ-23NPP, Laurell). Devices were patterned by removing excess P3HT from areas surrounding the channel, then leaving them in a vacuum oven overnight at 50 °C to remove any residual solvent. The devices were tested in a nitrogen environment using an Agilent 4155C semiconductor parameter analyzer. The field-effect hole mobility was calculated under transistor operation (VDS = −80 V; VG = 80 to −80

3. CONCLUSIONS The results presented here show that P3HT fiber length plays an important role in the formation of birefringent (liquid crystalline) fluids of P3HT solution with long-range order as well as in the formation of anisotropic thin films. Although sonication alone produces shorter fibers that have isotropic orientation both in the solution and thin film states, aging for a sufficient duration can lead to longer fibers that provide for orientational order both in solutions and thin films. When inducing aggregation by both sonication and aging, the sequence of techniques plays an important role. Sonication of aged solutions composed of longer fibers effects breakup of the fibers into smaller segments, with concomitant increase in voidfilled grain boundaries and lower charge carrier mobility. In contrast, sonication prior to aging affords the highest levels of aggregation because of accelerated nucleation and growth. The combined sonication/aging technique gives rise to samples that exhibit the highest charge carrier mobility (approximately 0.15 cm2 V−1 s−1) among the alternatives explored here. This value represents an 11-fold increase over pristine P3HT and is significantly higher than the mobility achieved by sonication or aging alone. The sonication process resulted in a delay in the onset of birefringence. Presumably, the presence of smaller G


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Chemistry of Materials Notes

V) by plotting the square root of the drain current (ID) versus gate voltage (VG) and obtaining the slope to extract the mobility according to the following equation:45 ID = μC0

W (VG − VT)2 2L

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation (CBET 1264555), the Georgia Institute of Technology, and the Brook Byers Institute for Sustainable Systems is gratefully acknowledged. N.K. and M.A.M. thank the NSF NESAC IGERT (DGE-1069138) program for Traineeship support; N.P. thanks the NSF FLAMEL IGERT (1258425, IGERTCIF21) program for Traineeship support. We also appreciate time spent with Mohan Srinivasarao and Mincheol Chang in helpful discussions.

(2)

where W (2000 μm) and L (50 μm) are the transistor channel width and length, respectively, VT is the threshold voltage, and C0 is the capacitance per unit of the SiO2 gate dielectric (1.15 × 10−8 F cm−2). For each solution condition, four devices were measured to obtain average mobility values. 4.4. UV−Vis Spectroscopy. An Agilent 8510 UV−vis spectrometer was used to record the absorption spectra of films (spin-coated on precleaned glass slides at 800 rpm for 30 s) and solutions (placing droplet between glass slide and coverslip and securing with clips). 4.5. Atomic Force Microscopy. AFM measurements were performed on the same devices used for OFET measurements, using an ICON Dimension scanning probe microscope (Bruker) in tapping mode with a silicon tip (NSC-14, MikroMasch). 4.6. Polarized Optical Microscopy. POM images were obtained using a Leica DMRX optical microscope equipped with a rotatable polarizer and analyzer and a rotatable stage. Images were captured using a Nikon D300 digital SLR camera. 4.7. Raman Micropectroscopy. Raman spectra were obtained using a 50× objective and a 785 nm laser light source (Kaiser OpticSystem) that has 4 cm−1 resolution in the backscattering geometry. Spectra were acquired at a laser power of 20 mW with 6 s exposure time and 6 accumulations each. For each spectrum, the peak corresponding to CC stretching was fit to a Lorentzian function using Holograms software to obtain peak heights. 4.8. Synchrotron Radiation Characterization. GIWAXS measurements were carried out on beamline 11−3 at the Stanford Synchrotron Radiation Light Source (SSRL). The beam was kept at an energy of 13 keV and the critical angle of measurement was 0.12°. A LaB6 standard sample was used for calibration. Using the calibration, wavelength, and sample−detector distance (400 mm), the 2D images were corrected from intensity versus pixel position to intensity versus q-spacing using WxDiff software. 2D images were reduced to 1D plots via integration of cake segments and analyzed using Origin Pro software for peak fitting.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01163. Additional POM images including capillaries and films at multiple angles, GIWAXS images for all samples, sample of fitting of 1D plot of GIWAXS data, transfer curves of OFET data swept in forward and reverse directions, output curves for sample OFET device, plots of mobility as a function of eight structural parameters, persistence lengths of nanofibers calculated from AFM images using FiberApp software, examples of spectra for Raman anisotropy, all UV−vis curves, and examples of UV−vis fits to obtain percent aggregates and exciton bandwidths. (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address

G.W.: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States. H


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